One known arrangement of the air conditioning system of this type is to perform the interior heating by introducing the high temperature cooling water after cooling the engine into a heater core to transfer the heat from the water to the air of the interior, as disclosed in the following patent document 1, for example. In this system, by means of a high flow type water pump it is possible to feed a sufficient amount of cooling water to a heater core, so that a sufficient quantity of heat for the interior heating can be obtained.    Patent document 1: JP-A-H04-358921
In such a vehicle air conditioning system, however, when the cylinder head and cylinder block are cooled, a part of the cooling water is vaporized and the produced bubbles are transferred to the heater core. As a result, the system has a problem that a drop in efficiency of heat exchange and the noise of interior heating are caused in the heater core.